Brazed aluminum equipment is subject to the severe problem of intergranular corrosion in corrosive environments on surfaces coated with the brazing alloy. The corrosive environments which can cause this problem include water containing dissolved chloride, bicarbonate or sulfate ions, especially if the pH of the water has a relatively low value. Such waters may condense as films on the fins of heat exchanger equipment used for automotive or aircraft air conditioners, automotive radiators, gas liquefaction equipment or the like.
Intergranular corrosion has also been encountered in other applications, as on brazed headers inside automotive radiators and heat exchangers generally. In such cases, the coolant is usually corrosive. For example, if automotive antifreeze solutions are used, poor maintenance can often result in the solution becoming corrosive for a variety of reasons. Chief among these reasons are that the antifreeze may have been allowed to remain in the radiator for a number of years without replacement while replenishing the level with mixtures of fresh antifreeze solution with hard natural water. These practices would deplete the corrosion inhibitors and reserve alkalinity components, permitting the coolant pH to drop and allowing heavy metal ions to accumulate from reaction of the acids with copper alloy and cast iron surfaces in the coolant system.
U.S. Pat. Nos. 3,898,053 and 3,853,547 describe certain aluminum-silicon brazing compositions for joining aluminum alloy components; however, these compositions do not solve the problem of intergranular corrosion described hereinabove.
The problem of intergranular corrosion may occur whether flux brazing or vacuum brazing techniques are employed. There is evidence in the case of flux brazed aluminum Alloy 3003 (an aluminum base alloy containing from 0.05 to 0.20% copper, from 1 to 1.5% manganese, up to 0.6% silicon and up to 0.7% iron) clad with aluminum Alloy 4343 (an aluminum base alloy containing from 6.8 to 8.2% silicon, up to 0.8% iron, up to 0.25% copper, up to 0.1% manganese, up to 0.2% zinc and the balance essentially aluminum) that the silicon rich eutectic formed when the Alloy 4343 brazing alloy is brazed can migrate into the grain boundaries of the Alloy 3003 component and can cause increased susceptibility to intergranular corrosion. A similar silicon rich eutectic migration into the parent metal can occur in the case of vacuum brazed assemblies made from aluminum Alloy 3003 clad with the silicon rich aluminum vacuum brazing Alloy MD 150 (an aluminum base alloy containing about 9.5% silicon, 1.5% magnesium, up to 0.3% iron, up to 0.05% copper, up to 0.07% manganese, up to 0.01% titanium and the balance essentially aluminum) or aluminum vacuum brazing Alloy MD 177. The MD 177 alloy has substantially the same composition as MD 150 containing in addition from 0.08 to 0.1% added bismuth. In both MD 150 and MD 177 the magnesium addition is used to getter traces of oxygen in the vacuum brazing furnaces.
Flux brazed assemblies made from No. 12 brazing sheet (aluminum Alloy 3003 clad on both sides with aluminum Alloy (4343) and monolithic aluminum Alloy 3003 components are sensitized to corrosion by prolonged holding at elevated temperatures below the brazing temperature. This practice is used to assure that the final relatively short time brazing step will liquify the brazing alloy everywhere in very large assemblies. The effect of this holding time, which may be up to 5 hours at 1000.degree. F for large gas liquifaction heat exchangers, is to coarsen cathodic iron rich second phase particles in the metal. This causes increased susceptibility to both intergranular corrosion and pitting corrosion.
Accordingly, it is a principal object of the present invention to provide an improved brazed aluminum composite which is characterized by substantial resistance to intergranular corrosion.
It is a still further object of the present invention to provide an improved brazed aluminum composite as aforesaid which is inexpensive and convenient to use on a commercial scale.
Further objects and advantages of the present invention will appear hereinbelow.